* Election-year pressures weigh on Obama nuclear agenda
* Nuclear security that he championed faces budget cuts
* Iran, N. Korea standoffs could overshadow Seoul summit (Adds U.S. officials' comments, planned DMZ visit)
By Matt Spetalnick and Alister Bull
WASHINGTON, March 20 (Reuters) - Shortly after taking office, U.S. President Barack Obama set the goal of eventually ridding the world of nuclear weapons as a central theme of his presidency and pledged dramatic steps to lead the way.
But after plenty of lofty rhetoric and a few steps forward, Obama is facing fresh doubts about whether he is willing to take the political, diplomatic and budgetary risks needed to bring his vision closer to reality.
At home, arms-control advocates who once extolled his ambitious plans are complaining about funding cuts for several key nuclear security programs, while critics on the right are leveling election-year accusations that his policies weaken America's strategic deterrence.
At the same time, Obama's efforts to spur global cooperation to prevent nuclear terrorism, which he once called the "single biggest threat" to U.S. security, have been overshadowed by the more urgent challenges of thwarting Iran's and North Korea's nuclear development.
All of this is weighing on Obama's broader nuclear agenda as he heads to Seoul next weekend for the second Nuclear Security Summit. He inaugurated the event earlier in his tenure, and aides say he is committed to keeping the process on track.
"The nuclear-free vision thing has run up against facts on the ground," according to an outside expert who advises the White House on national security. "So, for now, there's going to be an abundance of talk and not much serious action."
Granted, it's been more than just talk up to this point.
Obama unveiled a revamped policy in 2010 renouncing development of new nuclear weapons and restricting use of those already in Washington's arsenal. He followed that up by signing a landmark arms reduction treaty with Russia last year.
He secured commitments from world leaders at the inaugural 2010 summit in Washington to help keep bomb-grade material out of terrorists' hands, and independent experts say most of the pledges are being met - though many were modest in scope.
But momentum seems to have slowed on Obama's nuclear agenda and, with the Nov. 6 presidential election looming, chances for major new advances look doubtful.
Underscoring a sense of caution, defense and national security officials have spent months debating a secret set of new options being prepared for Obama to help guide future arms-control talks. Ideas range from maintaining the status quo to reducing warheads by up to 80 percent, an official has said.
But the administration appears reluctant to push publicly on such a divisive issue as his re-election campaign gathers pace.
While some in Obama's liberal base are disappointed he has not done more, conservatives see his nuclear policies as a point of vulnerability that Republican candidates can exploit.
"Instead of dealing with real nuclear threats like Iran and North Korea, he's going to magic shows and talking about a world without nuclear weapons, which would be a much less safe world for the United States," said John Bolton, who was U.N. ambassador under Obama's Republican predecessor, George W. Bush. The hawkish ex-official has endorsed Mitt Romney for the Republican presidential nomination.
Meanwhile, anti-proliferation groups, which credit Obama with raising the global profile of nuclear security, are also voicing complaints. They are unhappy that the fiscal squeeze in Washington has translated into reduced spending on several nuclear security programs in his budget plan for fiscal 2013.
Ken Luongo, president of the Partnership for Global Security, a non-governmental research organization, called cuts in Obama's budget request to secure weapons of mass destruction "an assault on common sense."
Two programs at the Department of Energy's National Nuclear Security Administration are being pinched. Funding for the Global Threat Reduction initiative, designed to secure nuclear material at civilian sites around the world, was cut by $32 million to $466 million, and will be down $500 million over the next four years compared with the levels envisaged a year ago.
A bigger hit was taken by the International Nuclear Materials Protection and Cooperation program, designed to improve security at vulnerable stockpiles of nuclear weapons in countries deemed to be of special concern. Its funding request was trimmed by $259 million in 2013 to $311 million.
Laura Holgate, an Obama adviser on weapons of mass destruction, said some spending reductions reflect completion of specific projects in places like Russia. "I would certainly not suggest that any changes in the resourcing in any way reflect any changes in our commitments," she said.
While arms-control groups sees some progress on nuclear security due to Obama's efforts, they caution against complacency when more than 50 leaders meet on Monday.
Outside experts are mostly skeptical of the chances for meeting the Washington summit's headline pledge to safeguard all of the world's nuclear materials within four years and are now pushing for voluntary arrangements to be made enforceable.
Despite that, Obama will be able to tout U.S. strides against the threat of nuclear terrorism, including the killing of Osama bin Laden, who "famously articulated al Qaeda's desire to pursue a nuclear weapon," White House aide Ben Rhodes said.
U.S. officials also expect a positive scorecard on other countries in addition to pledges for further action. But no major breakthroughs are likely.
Iran and North Korea are not on the guest list or the agenda. But on the sidelines, Obama will urge Russia and China to help ratchet up pressure with sanctions and diplomacy. He will visit South Korea's tense border with the North on Sunday.
But Obama is expected to keep a cautious line with countries in attendance, including Pakistan, whose ties with Washington are strained. Experts see Pakistan as the biggest area of risk because it has a large stockpile of weapons-grade material and faces internal security threats from militant groups.
Obama set expectations high in a 2009 speech in Prague when he declared it was time to seek "a world without nuclear weapons." He acknowledged it was a long-term aspiration, but his high-flown oratory helped him win the Nobel Peace Prize.
Taking that vision much farther will not be easy.
With Republican opposition as strong as ever to the United States joining the global nuclear test-ban treaty, Obama for now has had to shelve his earlier promise to push for ratification.
Another arms accord with Moscow will be an even tougher sell to conservatives who say Obama he has not moved fast enough to modernize the U.S. strategic arsenal, a pledge he made in return for Republican votes that helped ratify the START treaty.
"Their commitment to it (nuclear modernization) was weak ... It turns out at the first challenge they folded," said U.S. Senator Jon Kyl, a leading Republican on nuclear issues.
Meantime, the ultimate test of Obama's nuclear agenda could well be what happens in coming months with Iran. Its development of nuclear arms - something it denies it seeks - could spark war with Israel and an arms race in the Middle East. (Additional reporting by Caren Bohan, Susan Cornwell and David Alexander; Editing by Paul Simao)